Add benchmark for writeBlockHeader.
Add benchmark for writeBlockHeader.

This commit adds a benchmark for the writeBlockHeader function.

1d4Nf6/btcd,davecgh/btcd,soapboxsys/ombfullnode,ltcsuite/ltcd,qchain/btcd,ltcsuite/ltcd,jongillham/btcd,dan-da/btcd,runeaune/btcd,adiabat/btcd,Cleawing/btcd,FactomProject/btcd,runeaune/btcd,decred/dcrd,FactomProject/btcd,DanielKrawisz/btcd,Roasbeef/btcd,dan-da/btcd,danoctavian/btcd,btcsuite/btcd,cjepson/btcd,tb00/btcd,jimmysong/btcd,jongillham/btcd,btcsuite/btcd,cqswxj2013/btcd,hectorj/btcd,Cleawing/btcd,jrick/btcd,dajohi/btcd,jcmincke/dcrd,wallclockbuilder/btcd,jrick/dcrd,stevenroose/btcd,adiabat/btcd,Dirbaio/btcd,davecgh/btcd,BitGo/prova,jimmysong/btcd,CryptocurrencyCabal/htcd,stevenroose/btcd,soapboxsys/ombfullnode,Roasbeef/btcd,decred/dcrd,DanielKrawisz/btcd,dajohi/btcd,jrick/dcrd,ppcsuite/ppcd,btcsuitereleases/btcd,ppcsuite/ppcd-old,danoctavian/btcd,cjepson/btcd,aakselrod/btcd,hectorj/btcd,toddfries/btcd,glv2/ppcd,tb00/btcd,jcmincke/dcrd,ppcsuite/ppcd-old,ppcsuite/ppcd,Dirbaio/btcd,mathiasrw/btcd,Dirbaio/btcd,Stash-Crypto/btcd,toddfries/btcd,qchain/btcd,cqswxj2013/btcd,aakselrod/btcd,jrick/btcd,jrick/btcd,btcsuitereleases/btcd,1d4Nf6/btcd,BitGo/prova,mathiasrw/btcd,wallclockbuilder/btcd,Stash-Crypto/btcd,dajohi/btcd,glv2/ppcd,jcmincke/dcrd,pkothbauer/btcd,pkothbauer/btcd,CryptocurrencyCabal/htcd